The invention relates to a process for cooling a molten film which is extruded from a slot die into a cooling system and stretched between the slot die and the cooling system, and also to an apparatus for implementation of the process.
The production of biaxially stretched flat films usually includes the following process steps: extrusion, molten film formation, molten film cooling to give the intermediate film, biaxial stretching with fixing and rolling-up of the film.
For instance, German Offenlegungsschrift No. 31 24 290 describes the production of a biaxially oriented polypropylene film which is extruded when molten and in web form, and which is subsequently cooled and solidified. The formed polypropylene film is preheated and oriented in the longitudinal direction and subsequently in the transverse direction. The longitudinal orientation takes place in this case in an at least a two stage process.
European Patent Application No. 0 026 911 discloses a process for the production of a biaxially stretched polypropylene film in which a longitudinal/transverse/longitudinal flat stretching process is carried out, to produce a longitudinally/transversely stretched polypropylene film with a double refraction value of between 0.010 and 0.030. This film is then restretched further in the longitudinal direction by 1.8 to 3.0 times its original length, the degree of necking being kept below 20%.
European Patent Application No. 0 115 917 describes a process for the production of a polypropylene film which is longitudinally stretched after extrusion from a die in the longitudinal direction by a ratio of 4 to 9 relative to its original length and the surfaces of the intermediate film are subsequently heated in order to bring the molecular chains in the surface layers into a nonoriented state. Thereafter, the film is restretched at a ratio of 1.2 to 3.
The process described in German Patent No. 15 04 454 for the production of shrinkable films from polypropylene provides for the extruded film to be stretched in the longitudinal direction to 5 to 7 times the original length and in the transverse direction to 8 to 13 times the original width, the second stretching taking place at a higher temperature than the first stretching. In heating the film to the longitudinal stretching temperature, the film is preheated to a temperature of between 125.degree. and 140.degree. C., then further heated to a temperature of between 140.degree. and 150.degree. C. while being simultaneously longitudinally stretched. Directly after the longitudinal stretching, the film is cooled, then heated to a temperature of between 150.degree. and 165.degree. C. and transversely stretched at a temperature of 150.degree. to 160.degree. C.
In the process step of molten film cooling to give the intermediate film, drum cooling is, at present, used virtually exclusively. In this technique, the molten film issues from a slot die and passes onto the surface of a cooled rotating drum. The following are typical data for the molten film and the take-off drum:
Width of the molten film: 0.5-2.0 m PA0 Thickness of the molten film: 0.3-3.5 mm PA0 Temperature of the molten film: 250.degree.-300.degree. C. PA0 Drum diameter: 0.5-2.0 m PA0 Circumferential speed of the drum: 20-60 m/min.
To achieve a good thickness profile and to avoid swelling of the extrudate, which entails the risk of surface defects, the molten film is stretched between the die and the drum. Typical stretching ratios are approximately 1.2 to 4.0 with polypropylene and approximately 4 to 25 with polyethylene terephthalate, the larger stretching figures relating to thinner molten film thicknesses. Where the molten film is fed onto the drum, it must be ensured that the reduction in width or the degree of necking caused by the elongation is kept small and that as little air as possible is drawn in between the molten film and the drum surface. A small degree of necking and a small amount of air intake can only be achieved by small distance between the die and the point of contact of the molten film, together with a suitable feeding method. Examples of suitable feeding methods are the air knife method, the suction box method and feeding with the aid of electrodes by the so-called pinning method.
The drum must fulfill two main requirements; namely, that it cool the molten film as effectively and evenly as possible and that it produce a satisfactory film surface.
To achieve an effective, i.e., rapid, cooling of the molten film, the heat transfer from the molten film to the drum must be as great as possible. This is achieved, for example, by cooling coils which are attached spirally to the inner surface of the shell of the take-off drum and through which cooled water flows. Another possibility is for the inner shell surface of the take-off drum to be sprayed with water over the entire circumference or part of the circumference (See, for example, European Patent Application No. 0 164 912).
The water/inner drum shell heat transfer is very intensive, with heat transfer coefficients (HTC) of up to .alpha..sub.WS .apprxeq.3500 W/m.sup.2 K being attainable here. Owing to the thermal resistance of the metal shell and the intermediate layer of drum surface/film, the heat exchange between cooling medium and film is reduced, so that in practice values for the transient coefficient K.sub.WF (cooling medium to film) in the range from 500 to 1500 W/m.sup.2 K are attained. In the production of polypropylene films, it has been found that the shorter the cooling time and the lower the film temperature reached during cooling, the better the optical properties of the film, such as gloss or dullness. The temperature of the intermediate film will be markedly less than 95.degree. C. during this cooling.
The cooling time, which lasts until a desired temperature of the intermediate film is reached after the cooling operation, depends on the material, on the heat transfer coefficients of the film and on the thickness of the molten film. With known material, and a predetermined configuration of the take-off drum and thickness of the molten film, the variation in temperature in the film over time is fixed and is virtually non-susceptible to further influence.
The final temperature in the intermediate film, which hereafter always refers to the temperature of the film after leaving the cooling system, is thus solely a function of the dwell time of the molten film on the take-off drum.
With a given circumferential speed v.sub.A of the take-off drum, the dwell time t.sub..omega. of the film on the take-off drum is associated with the diameter D via the following equation: t.sub..omega. =.pi..alpha.D/360.times.v.sub.A. The angle .alpha. specifies here the angle of inclusion of the film on the drum.
According to this equation, for a constant circumferential speed v.sub.A, a large dwell time can be achieved by a large diameter of the take-off drum. The solution which suggests itself is that of enlarging the drum diameter; however, it should be borne in mind that an increase in diameter is accompanied by disadvantages, such as, for example, inferior control over the process in terms of the concentricity characteristics of the take-off drum, temperature uniformity, rotary oscillation behavior, reduced space for the suction box feeding, greater traveling distance of the molten film in the case of air knife feeding and, above all, the risk of the molten film prematurely coming off the drum surface. This greatly reduces the heat transfer for the remaining cooling zone. Owing to this effect, the drum diameter must be increased beyond the design value, which can constitute a diameter increase of up to 50% in the case of large drums (D.gtoreq.1.5 m).